Black pigments based on Fe.sub.3 O.sub.4 have been successfully used for many applications. On account of their poor heat resistance, however, they are unsuitable for applications in autoclave-cured building materials or for temperature-resistant paints because they are oxidized to brown or red Fe.sub.2 O.sub.3.
Commercially available black pigments of the Fe.sub.2 O.sub.3 --Mn.sub.2 O.sub.3 system, as described in U.S. Pat. No. 3,615,810 are highly temperature-resistant and are eminently suitable for the applications mentioned above. However, pigments such as these have coloring strengths of only 40 to 70% compared with commercially available pigments based on Fe.sub.3 O.sub.4 with the result that relatively large quantities of the pigments mentioned have to be added, for example to building materials, to obtain a deep black color impression.
U.S. Pat. 3,615,810 describes a process for the production of a temperature-resistant black pigment of the Fe.sub.2 O.sub.3 --Mn.sub.2 O.sub.3 system consisting of phases with the structures of hematite and the principal constituent of bixbyite. In this process, oxidic or oxide-forming starting materials of iron and manganese are mixed, mineralizers, such as sodium chloride, are added to the resulting mixture and the whole is calcined at temperatures of 800.degree. to 900.degree. C. It is not possible by this process to obtain pigments of much higher coloring strength than the pigment Bayferrox.RTM. 303T. The pigment Bayferrox.RTM. 303T is a commercially available temperature-resistant black pigment of the Fe.sub.2 O.sub.3 --Mn.sub.2 O.sub.3 system manufactured by Bayer AG.
DE-A 2 159 364 describes a process for the production of temperature-resistant black pigments with the structure of hematite. In this process, .beta.-FeOOH is calcined with manganese-containing compounds. Due to the chloride structurally present in the .beta.-FeOOH, considerable quantities of chlorine gas are released during calcination with manganese dioxide, which is a disadvantage for industrial-scale operation of the process. Although the pigments obtained by this process have very high coloring strength, they are undesirably tinged with a yellow-red color undertone. Accordingly, these pigments are unsuitable for obtaining a deep black color impression, for example for the production of slate-colored building materials or for deep black coil coating compositions.
According to U.S. Pat. No. 4,145,229, heat-stable black pigments are produced by calcination of mixtures consisting of iron sulfate and manganese-containing compounds. In view of the SO.sub.2 formed during the process, the waste gases have to be subjected to expensive desulfurization. Black pigments are obtained for manganese contents of 11.5 to 46%, corresponding to an iron-to-manganese ratio of 5-0.5:1, and preferably 14 to 23%. Accordingly, the process follows the teaching of U.S. Pat. No. 3,615,810, according to which the phase having the structure of bixbyite bears the pigment properties. For manganese contents of less than 11. 5%, corresponding to an iron-to-manganese ratio of &gt;5:1, brown pigments are obtained.
According to U.S. Pat. No. 3,655,418, the hydroxides of iron and manganese are precipitated from solutions in the presence of flocculating agents and subsequently calcined. On account of their quality, the precipitates are difficult to filter and wash. The resulting salt contents in the filter cake give rise to extensive sintering during the calcination process, so that furnace clinkers that are difficult to grind up are obtained. The pigments lack coloring strength and are distinctly brownish. In addition, the decomposition of the salts during the calcination process means that the waste furnace gases have to be expensively purified.
According to U.S. Pat. No. 3,276,894, brown or black iron oxides of hematite structure are obtained by calcination of precursors produced by the aniline process. The black pigments obtained by this known process are distinguished by a distinct red tinge and by poor coloring strength. Their coloring strength by comparison with Bayferrox.RTM. 303T is only 40 to 50%.
Accordingly, the problem addressed by the present invention was to provide a heat-stable black pigment of high coloring strength which would not have any of the described disadvantages.